1. Technical Field
This invention relates to fine particles of polyvinyl chloride and a process for producing the same.
Plastisols and organosols of polyvinyl chloride (hereinafter sometimes referred to as PVC) are useful materials widely employed in various fields according to the methods of their use such as coating, slush molding, and dipping. In such applications, a PVC plastisol is required to have excellent viscosity characteristics such as low initial viscosity and no change in viscosity with elapse of time and excellent gelling characteristic. Also the product prepared therefrom is required to have excellent thermal stability, transparency and resistance to water.
A PVC plastisol is produced by dispersing particulate PVC in a plasticizer. In order to obtain a plastisol having excellent viscosity characteristics as mentioned above, it is desirable that the particulate PVC be constituted of particles which are all spherical with relatively large particle sizes and have an appropriate particle size distribution. These particles should also be closely packed.
In order to meet these requirements, it is considered necessary to produce a PVC comprising fine spherical particles of a particle size of less than 2 microns with small volume of voids which do not easily absorb a plasticizer at room temperature.
2. Prior Art
In the prior art, PVC generally employed for such uses has been produced according to the seed polymerization method by emulsion polymerization and the microsuspension method. However, the former method is cumbersome in its polymerization operations and control of the polymerization is difficult. Furthermore, the reproducibility is poor. On the other hand, the latter method has certain advantages such as the ease with which the particle sizes and their distribution can be controlled and the great degree of freedom in the choice of the emulsifier but also has a disadvantage in that a homogenizer of high pressure and high shearing force is required. A disadvantage common to these methods is that, since the polymer is formed as an aqueous dispersion, no particulate product can be obtained without recourse to a measure such as spray drying. The total energy cost necessary for removal of water or drying must be considered as a great problem in these days when the cost of fuel is ever increasing.
In this connection, it is to be remembered that there is a bulk polymerization method for production of PVC. According to this method, since the PVC formed is obtained as a dispersion in the liquefied monomer, release of the pressure after polymerization will immediately give a dry particulate product, thus overcoming all of the problems as mentioned above. However, the particulate PVC produced according to the practically applied bulk polymerization method, although having the advantage of skinless property, is porous in nature and absorbs a plasticizer, which makes it unsuitable for use in plastisols.
Concerning this point, some assert that fine particles of PVC with particles sizes of 10 to 50 microns having a narrow particle distribution can be produced by the bulk polymerization and are useful for preparation of plastisols (as stated in Japanese Patent Publication No. 25863/1972 and Japanese Laid-open Patent Publication No. 26278/1973). However, perusal of these prior-art disclosures reveals that the PVC thus produced is used in combination with the PVC for a plastisol as conventionally produced for control of the viscosity of the plastisol formed.
It is also known to carry out bulk polymerization in the presence of a cellulose derivative (as described in Japanese Laid-open Patent Publication No. 137289/1978). However, this method belongs to the category of the conventional bulk polymerization method in which the polymerization system is stirred in a conventional manner, and it appears to be intended to obtain a bulk polymerized product with a single particle size by the presence of a cellulose derivative. In fact, according to a trace experiment we conducted, agglomeration of particles or secondary particle formation occurred under such conditions, whereby no PVC with low plasticizer absorbability could be obtained.